1. Field of the Invention
This invention relates to an image exposing system for exposing an original image to an image forming surface, and, in particular, to a slit-scanning type image exposing system particularly suited to be used for slit-exposing an original image to a uniformly charged photosensitive member to form an electrostatic latent image corresponding to the original image thereon by selectively dissipating the charges.
2. Description of the Prior Art
A slit-exposing system for exposing a stationarily held original to a photosensitive member, which is being moved in a predetermined direciton, through an exposure optical system having movable parts by slit-scanning the original thereby moving the projected position in the direction opposite to that of the photosensitive member is known as described in the Japanese Patent Application No. 56-84182, which has been assigned to the assignee of the instant application.
Such a slit-exposing system will be briefly described hereinbelow with reference to FIG. 1. As shown, an original document 0 is stationarily placed on a transparent platen ( not shown ), under which a photosensitive member 2, which may be either in the form of a sheet or in the form of a belt, is moving to the left as indicated by the arrow at constant speed. Interposed between the original 0 and the photosensitive member 2 is an exposure optical system including movable lens 1 and a lamp 3. The two extreme positions of the lens 1 and the lamp 3 are indicated by the solid and dotted lines, between which the lens 1 and the lamp 3 may move in a reciprocating manner. When slitting-scanning the original 0, the lens 1 and the lamp 3 move from their respective positions indicated by the solid lines to the positions indicated by the dotted lines at constant speed. Then, upon completion of slit-scanning, or exposure, they return to their initial positions to be ready for the next operation.
At the initiation of a slit-scanning operation, since the lens 1 and the lamp 3 take the position indicated by the solid line, that portion of the original image 0 which is present at a slit-scanning start position A is focused onto an exposure start position A' on the photosensitive member 2. On the other hand, at the time of completion of the slit-exposure operation, the lens 1 and the lamp 3 each take the position indicated by the dotted line, so that that portion of the original 0 which is present at a slit-scanning termination position B is focused onto an exposure termination position B'. One of the features of this exposure system resides in the separation in position between the exposure start and termination positions with the exposure start position A' defined downstream of the exposure termination position B' with respect to the moving direction of the photosensitive member 2, whereby, during the slit-exposure operation, the position of projection of that part of the original 0 which is slit-scanned moves in the direction from the position A' to the position B', i.e., in the direction opposite to the moving direction of the photosensitive member 2, at constant speed.
As shown in FIG. 1, denoting the slit-scanning length by 1, the distance between the exposure start and termination positions A' and B' by L, the magnification rate by m, and the speed of the lamp during slit-exposure operation by v, then, in order to carry out the above described slit-exposure properly, the lens 1 and the photosensitive member 2 must be moved at the following speeds v.sub.L and v.sub.S, respectively. EQU V.sub.L =(ml-1)/(1+m))(1/1)v EQU V.sub.S =((m1-L)/1)v
At the time of completion of the slit-exposure operation, the trailing end of the latent image thus formed is located at the position B', so that by having the lens 1 and the lamp 3 returned to their original positions indicated by the solid lines during the time period in which the trailing end of the latent image formed on the photosensitive member 2 reaches the position A', the next exposing operation may be started to form another latent image in contiguous with the trailing end of the last preceding latent image. This is advantageous because latent images may be formed on the photosensitive member 2 successively without a gap between the images.
However, in carrying out slit-exposure, in general, it is often times required to apply some measure for eliminating the effect of stray light and to provide an exposure amount control unit, such as a slit plate which is provided with a slit of a predetermined size extending in the direction perpendicular to the moving direction of the photosensitive member 2, for controlling the amount of exposure light to be uniform across the image forming area where a slit-scanned portion of the orignal image is focused. In the case of an exposure system such as the one shown in FIG. 2, a slit plate 4 may be movably provided in the vicinity of the orignal 0, or, alternatively, a slit plate 5 may be movably provided in the vicinity of the photosensitive member 2. In either case, however, the slit plate 4 or 5 must be moved in synchronism with the slit-exposure operation, and thus the provision of a driving source therefor is required, which then tends to make the entire system complex in structure and bulky.